A magnetic disk drive, such as a hard disk drive, stores data on one or more disks coated with a magnetic medium. For read/write purposes, the surface of the magnetic medium carries a number of generally parallel data tracks, which on a disk type medium, are arranged concentrically with one another about the center of the disk.
An actuator arm positions a transducer or “head” over a desired track, and the head writes data to the track or reads data from the track. As the disk rotates, the actuator arm moves the head in a radial direction across the data tracks under control of a closed-loop servo system, based on position information or “servo data”, which is stored within dedicated servo fields of the magnetic medium of the disk. The servo fields can be interleaved with data sectors on the disk surface or can be located on a separate disk surface that is dedicated to storing servo information. As the head passes over the servo fields, it generates a readback signal that identifies the location of the head relative to the center line of the desired track. Based on this location, the servo system moves the actuator arm to adjust the head's position so that it moves toward a position over the desired track and/or a desired location within the track of current interest.
One requirement in the manufacture of such a hard disk drive relates to the formation of the servo patterns on the magnetic disk, which must be in concentric circular patterns. Systems for forming the servo tracks on magnetic disks have used both stepped translation mechanisms with laser beams and continuous translation mechanisms with electron beams.
FIG. 10 is a simplified diagram useful in explaining a prior technique for forming the concentric servo track patterns, using a rotating turntable and a beam jogging mechanism. During processing, the disk 1 rests on a turntable (not separately visible in the illustrated orientation) that rotates about the axis of the turntable and the disk but is otherwise stationary. The stationary turntable rotates continuously in the direction of arrow A, thereby rotating the disk under movable laser beam, represented by the circular spots 3. The translational position of the beam along the disk radius remains stationary, for one rotation while a circular exposure is made. The beam may be modulated during the exposure cycle. As a result, the beam forms servo signals along one of the circular tracks 15 (represented by solid lines), as the disk rotates through one revolution. Then the beam is turned off (as represented by the intermediate square spots between track circles), and during the next disk rotation, the translation mechanism jogs the laser beam to the position for the next track. This process, of applying the beam to form servo signals on one track during one rotation, and then jogging the beam to its next position while the beam is off during the subsequent rotation, repeats through successive alternate disk rotations, until all of the concentric servo tracks 15 have been formed on the disk 1.
This system of jogging the laser beam during alternate rotations requires a mechanical arrangement to move the laser beam or relevant components of the beam optics. The jogging mechanism may be mechanically complex, and it takes time to jog the beam to the next active spot 3. One disadvantage is that two rotations are needed for each recording pass, one pass for the actual servo track recording and one pass to jog the beam to the new position before the start of recording of the next servo track. Consequently, the formation of the desired servo patterns takes an excessive amount of time. A second disadvantage of existing systems of this type is that the translation mechanism, to jog the beam, does not allow for micro-stepping of the beam, hence, the equipment imposes a limit on the possible spacing between the circular servo patterns.
An alternate approach, developed for use with an electron beam (although equally applicable to a laser beam) and using movement of the turntable, produces a spiral or helical pattern, not a pattern of concentric circles. FIG. 11 shows the translation in such a system, and FIG. 12 shows the resulting spiral servo pattern. Again, during processing, the disk rests on a turntable (not separately visible in the illustrated orientation). In this case, however, the beam remains stationary at spot 5. In the example, the turntable rotates in the direction indicated by the arrow B (although it could also rotate in the opposite direction), but here, the turntable also moves laterally in the direction of translation represented by the arrow T. With such disk translation, at the start of processing, the disk will be in the position represented by the circle 7. As the turntable moves the disk in the translation direction T, the disk moves until it approaches and reaches it's ending position, as represented by the circle 9.
Throughout the process, the turntable continuously rotates and translates the disk under the stationary electron beam at spot 5. The electron beam approach does not require interruption to step the translation. Also, this technique can produce smaller variations in the translation and thus smaller spacings between turns of the continuous servo pattern. However, the continuous rotation and translation results in recorded spirals 13 on the disk 11, essentially winding from a start point near the periphery of the disk to a point near a central opening 17, as shown in FIG. 12. As noted above, servo patterns on magnetic disks require concentric circles 15 (see FIG. 10). Hence a need exists for a technique to adapt the continuous translation approach (with the stationary beam) to produce a concentric circular pattern 15 required for servo regions of a magnetic disk for a hard disk drive or the like.